Method and apparatus for measuring amount of coating on paper web

ABSTRACT

A method and an apparatus for measuring the amount (CW) of a coating ( 2 ) on a paper web ( 1 ). The amount (CA) of at least one component in the coating ( 2 ) on the paper web ( 1 ) is measured and the composition (CC) of the coating ( 2 ) to be transferred to the paper web ( 1 ) is determined. The amount (CW) of the coating ( 2 ) on the paper web ( 1 ) is determined on the basis of the amount (CA) of at least one component in the coating ( 2 ) on the paper web ( 1 ) and the composition (CC) of the coating ( 2 ) to be transferred to the paper web ( 1 ).

FIELD OF THE INVENTION

The invention relates to a method of measuring the amount of a coatingon a paper web, in which method the amount of at least one component ofthe coating on the paper web is measured.

The invention also relates to an apparatus for measuring the amount of acoating on a paper web.

BACKGROUND OF THE INVENTION

Paper coating refers to its coating with various substances, the mostlyused being pigment-containing coating colours. Other generally usedcoatings include waxes, plastics, silicone, surface sizes and starch.The purpose of coating is to fill the irregularities on the papersurface with one or more coating layers. Coating primarily affects theprintability and appearance of paper, but also the stiffness of paperand its resistance to water, grease or solvents.

A coating is composed of an aqueous solution of one or more pigments,one or more binders, and additives. For some special papers,solvent-based coatings are used. Accordingly, pigments, binders andadditives constitute the components of a coating. A coating is preparedat a coating kitchen by mixing these components. The mixing can beperformed either in doses as batch production or by continuous mixing.The proportions or amounts of the different components are given incolour formulas. From the coating kitchen, the coating is transferred toa coating head, where it is evenly applied onto the surface of a paperweb. The thickness, i.e. amount (g/m²) of the coating layer is adjustedsuitable by peeling excess coating off the paper by means of a doctorblade. The amount of the coating is adjusted by changing the position ofthe doctor blade relative to the paper web. Excess coating is directedto a circulation, from where it is reapplied onto the surface of thepaper. The coating layer remaining on the surface of the paper is driedby evaporating the excess water contained by the coating with coatingdrying units located after the coating head. Either both or only oneside of the paper can be coated by using either separate coating headsor by coating both sides simultaneously. The coatings on the differentsides of the paper may also be identical or different depending on theunequal sidedness of the base or the operational asymmetry required bythe end product.

For adjustment of the amount of a coating on the paper, the amount ofthe coating remaining on the paper is measured and, based on saidmeasurement, the position of the doctor blade is changed relative to thepaper web, if required, to change the amount of the coating. Nowadays,the amount of a paper coating is measured by IR measurement. U.S. Pat.No. publication 4,957,770 discloses a method based on IR measurement forsensing the amount of a coating on a moving base. Measuring the amountof a coating by IR measurement is based on a measurement of the ratio ofthe reflection intensities of the IR absorption wavelength bands andreference wavelength bands characteristic of the different components ofthe coating and water and fibres. The ratio of the reflectionintensities of the absorption wavelength bands and the referencewavelength bands corresponding to the components enables thedetermination of the amount of each component in the coating remainingin the paper. The total amount of coating remaining in the paper can bedetermined based on the amount of a component, once the formula of thecoating applied onto the paper, i.e. the substances mixed into thecoating and the amounts thereof are known.

However, a problem in present solutions for measuring the amount of acoating is that the composition of the coating has to correspond to theformula exactly in order for the amount of the coating remaining in thepaper to be determined correctly. Because of the recycling of coating orproblems associated with its manufacture, the composition of the coatingmay change, i.e. the coating no longer corresponds to the formula. Whenthe coating does not correspond to the formula, the total amount of thecoating in the paper cannot be determined exactly. In batch processes,the composition of the coating may differ from what is stated in theformula for instance because of inaccuracy in metering the components ofthe coating, transition points occurring in grade changes or incompletewashing of coating treatment devices. In a continuous coating productionprocess, inaccuracy is caused for instance by inaccuracy in meteringmeasurements, unfamiliarity of recycled pigments precipitated in washingand grade changes.

BRIEF DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a new type of methodand apparatus for measuring the amount of a paper coating.

The method of the invention is characterized by determining thecomposition of the coating to be transferred to the paper web, anddetermining the amount of the coating on the paper web on the basis ofthe amount of at least one component of the coating on the paper web andthe composition of the coating to be transferred to the paper web.

The apparatus of the invention is characterized in that the apparatuscomprises a first measuring device arranged to measure the amount of atleast one component in the coating on the paper web by reflectionmeasurement, a second measuring device arranged to determine thecomposition of the coating to be transferred to the paper web, and adata processing device arranged to determine the amount of the coatingon the paper web on the basis of the amount of at least one component ofthe coating on the paper web and the composition of the coating to betransferred to the paper web.

The essential idea of the invention is to measure the amount of acoating on a paper web by measuring the amount of at least one componentof the coating on the paper web, by determining the composition of thecoating to be transferred to the paper web and by determining the amountof the coating on the paper web on the basis of the amount of at leastone component of the coating and the composition of the coating of thepaper web to be transferred to the paper web. According to an embodimentof the invention, the amount of at least one component of the coating onthe paper web is measured by reflection measurement based on infraredtechnique. According to a second embodiment of the invention, thecomposition of the coating to be transferred to the paper web isdetermined by reflection measurement based on infrared technique.According to a third embodiment of the invention, the composition of thecoating to be transferred onto the paper web is determined by Ramanspectroscopy based on molecular vibration spectroscopy. According to afourth embodiment of the invention, the amount of at least one componentof the coating to be measured is the amount of a pigment in the coatingon the paper web. According to a fifth embodiment of the invention, theamount of the coating on the paper web is adjusted on the basis of ameasurement of the amount of the coating on said paper web.

An advantage of the invention is that the exact amount of a papercoating is found out in all running conditions, and a deviation in thecomposition of the coating from the formula does not cause an error inthe measurement of the amount of the coating. The formula used in thepreparation of the coating does not have to be known, and grade changesdo not either cause problems in measuring the amount of the coating.When the composition of the coating to be transferred to the paper webis determined by reflection measurement based on infrared technique orby Raman spectroscopy based on molecular vibration spectroscopy, thecomposition of the coating can be determined rapidly during normalprocess operation without laboratory analyses causing long delays. Whenthe composition of the coating is determined by Raman spectroscopy, thecomposition of aqueous samples can also be measured extremelyaccurately, since water is a weak Raman scatterer. The amount of atleast one component to be measured of the coating on the paper web ispreferably the amount of a pigment in the coating, since the proportionof pigments in the dry matter of the coating is typically the highest.An exact measurement of the amount of the coating on a paper web furtherenables an accurate adjustment of the amount of the coating on the paperweb.

In the context of the present description, the term ‘paper’ refers toboth paper and paperboard.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described in detail in the attached drawings,wherein

FIG. 1 schematically shows a paper coating process and an apparatus formeasuring the amount of a paper coating,

FIG. 2 schematically shows a measuring device for measuring the amountof at least one component of a coating on a paper web,

FIG. 3 schematically shows a measuring result of the measuring deviceaccording to FIG. 2,

FIG. 4 schematically shows a measuring device for determining thecomposition of a coating to be transferred to a paper web, and

FIG. 5 schematically shows a measuring result of the measuring deviceaccording to FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows a coating process for coating a paper web 1moving in the direction of arrow A with a coating 2 or coating colour 2.Various pigment-containing coating colours or waxes, plastics, silicone,surface sizes or starch can be used as the coating 2. The coatingprocess includes a coating head 3, by means of which the coating 2 isconveyed, i.e. applied to the lower surface of the paper web 1. Thecoating head 3 includes a coating colour reservoir 4 or applicator pan4, an applicator roll 5, a backing roll 6, a doctor blade 7 or blade 7and a collector pan 8. The coating head 3 shown in FIG. 1 is a rollapplicator or roll coater, wherein the coating 2 is lifted in thecoating colour reservoir 4 to the lower surface of the paper web 1supported by the backing roll 6 by means of the applicator roll 5rotating in the direction of arrow B. The backing roll 6 naturallyrotates in the direction shown by arrow C. The amount of a coating 2conveyed by the applicator roll 5 to the surface of the paper web 1 isaffected by the size of an application gap 9 or nip 9 between theapplicator roll 5 and the backing roll 6, the properties of the coating2, the speed of the applicator roll 5, and the diameter and hardness ofthe applicator roll 5 and the backing roll 6. The amount of coatingtransferred from the coating colour reservoir 4 to the paper web 1 istypically about 200 to 250 g/m². After the coating 2 is applied, thecoating 2 is levelled or metered with the doctor blade 7, whereby excesscoating 2 is collected to the collector pan 8, from where it can againbe recycled for reuse. After the levelling, the amount of coatingremaining in the paper web 1 varies between 0.5 and 50 g/m². After thelevelling of the coating, a coating layer 10 remaining in the paper web1 is dried by removing the excess water conveyed to the web togetherwith the coating 2 using drying units 11 intended for drying thecoating. For the sake of clarity, FIG. 1 shows the paper web 1 and thecoating layer 10 remaining on top of it substantially thicker than theyreally are as compared with the structure of the coating process.

The coating process of FIG. 1 further comprises a coating preparationprocess 12 for preparing the coating 2. The coating 2 is typicallycomposed of an aqueous solution of one or more pigments, one or morebinders, and additives, but in association with some special papers,solvent-based coatings are used. The coating preparation process 12shown in FIG. 1 is a continuous process including a mixer 13, wherein apigment conveyed along a transfer line 14′ from a pigment tank 14, abinder conveyed along a transfer line 15′ from a binder tank 15, anadditive conveyed along a transfer line 16′ from an additive tank 16,and the excess coating returning along a recycling line 8′ from thecollector pan 8 of the coating head 3, are mixed together. The finishedcoating is conveyed from the mixer 13 along a feed line 13′ to thecoating colour reservoir 4. Pigments and additives are stored in thepigment 14 and binder 15 tanks as slurries mixed in water. For the sakeof clarity, FIG. 1 only shows one pigment tank 14, binder tank 15 andadditive tank 16, but it is apparent that the different pigments,binders and additives are stored each in a separate tank. The recyclingline 8′ also comprises different devices used for purifying the recycledcoating, but they are not shown for the sake of clarity. Further, forthe sake of clarity, FIG. 1 does not show the pumps and valves requiredin the transfer, recycling and feed lines for transfer and guidance ofboth the finished coating and the different components of the coating.

The pigments most generally used include kaolin and calcium carbonate.Other generally used pigments include talc, gypsum, titanium dioxide,aluminium hydroxide, and calcinated kaolin. The portion of pigment inthe dry matter content of the coating 2 is typically between 70 and 95%,and thus it mainly determines the quality and properties of the coating2. The binder serves to bind the pigment particles to each other and tothe paper web 1. Natural binders include starch, soybean protein andcasein. Synthetic additives include for instance different latexes, suchas styrene butadiene (SB) latex, and carboxy-methyl cellulose (CMC). Theportion of binders in the dry matter content of the coating is usuallybetween 5 and 25%. Additives are used to adjust different properties ofthe coating, such as viscosity and water retention. Urea or polyethyleneglycol, for example, can be used to lower the viscosity of the coating.The water retention of the coating can be adjusted with CMC or starch,for example. Other additives used in coatings include for instanceantifoam agents and foam inhibitors, lubricants, hardeners and opticalclarifying agents. The portions or amounts of components to be mixed inthe coating are given in formulas, wherein the amounts of components aregiven by denoting the total dry matter amount of the different pigmentsby the ratio 100, the portions of the different pigments being given asdry matter portions. The amounts of binders and additives are given asthe portion of their dry matter in the total dry matter of the pigments.The ratios of the different components of the coating vary depending onthe grade and purpose of use of the paper, and the coating method.

Paper coating and preparing a coating are known per se to a personskilled in the art, and they are thus not described in detail herein. Itis thus apparent that the operating principle of the coating head usedto coat paper may vary, i.e. instead of roll application, for instanceshort dwell application or nozzle application can be used, whereby theamount of the coating transferred to the paper web 1 at the coating headmay vary significantly. Furthermore, the coating may be prepared asbatch production instead of continuous preparation, whereby the coatingis transferred from the coating kitchen to a storage tank and from thereto a machine tank, from where the coating is pumped to the coatingcolour reservoir of the coating head.

For measuring the amount CW of a coating in the paper web 1, i.e. thetotal amount CW of the coating remaining in the paper web 1 afterlevelling, a first measuring device 17 is used to measure the amount CAof at least one component in the coating remaining in the paper web 1after the levelling of the coating, and a second measuring device 18 isused to determine the composition CC of the coating to be transferred tothe paper web 1, i.e. the different components of the coating and theiramounts or portions relative to each other. The amount CW of the coatingin the paper web 1 is found out by combining the amount of at least onecomponent in the coating measured with the first measuring device 17 andthe composition of the coating determined with the second measuringdevice 18.

The measurement of the amount of a coating in the paper web 1 on thebasis of said measurements can be illustrated by an example, wherein thefirst measuring device 17 is arranged to measure the amount of calciumcarbonate, the measuring result being 5 g/m². The second measuringdevice 18 was used to determine the composition of the coating,according to which the portion of calcium carbonate in the coating to beapplied to the paper web 1 is 60% of the total amount of the pigments.Accordingly, in accordance with the above description, the ratio ofcalcium carbonate is 0.6. Furthermore, the total ratio of the pigments,binders and additives contained by the coating, obtained on the basis ofthe composition of the coating measured with the second measuring device18 is 1.15. These values are used to calculate the amount CW of thepaper coating using the formula $\begin{matrix}{{CW} = {{\frac{5\quad{g/m^{2}}}{0.6} \times 1.15} = {9.6\quad{g/{m^{2}.}}}}} & (1)\end{matrix}$The amount CA of at least one component in the coating measured with thefirst measuring device 17 is combined with the composition CC of thecoating determined with the second measuring device 18 in a dataprocessing device 19, which is for instance a microprocessor-based orsignal processor-based device, which uses software to determine theamount CW of the paper coating. Instead of the data processing device 19determining the amount CW of the paper coating on the basis of theamount CA of at least one component in the coating and the compositionCC of the coating given directly by the measuring devices 17 and 18, thedata processing device 19 can be arranged to determine said quantitieson the basis of other measuring results obtained from the measuringdevices 17 and 18 and indirectly descriptive of said quantities.

The first measuring device 17 is arranged to measure the amount CA of atleast one component in the coating remaining in the paper using areflection measurement method based on the IR (infrared) technique. Thefirst measuring device 17 is arranged after the coating drying units 11.If the coating head 3 is directly integrated into a paper machine, thenthe first measuring device 17 is typically arranged in a measuringcarriage moving in a measuring frame immediately before the reeler ofthe paper machine, the sledge moving transversely to the paper web 1perpendicularly across the paper web 1 to and fro as the paper web 1continuously moves forward. The second measuring device 18 is arrangedto determine the composition CC of the coating to be applied to thepaper web 1 either as the first measuring device 17, using reflectionmeasurement based on IR technique or the CCD Raman technique. The secondmeasuring device 18 can be arranged in several points of the coatingprocess. FIG. 1 shows some potential points where the second measuringdevice 18 can be arranged. These points include the mixer 13, thecoating colour reservoir 4, and the feed line 13′ between them. In batchproduction of coating, the second measuring device 18 can be arranged inthe coating colour reservoir, in the storage or machine tank of thefinished coating or in the transfer line between them, or in the feedline between the machine tank and the coating colour reservoir. It isalso possible to arrange the second measuring device 18 by providing aseparate sample line for the second measuring device 18 for instance inone of the aforementioned tanks or the coating colour reservoir.

By determining the composition CC of the coating to be applied to thepaper and by comparing the amount CA of at least one component in thecoating, measured from the paper, with said composition CC of thecoating, the exact amount CW of the coating in the paper is found out inall running conditions. The formula used in the preparation of thecoating does not have to be known, and any disturbances in theproduction of the coating resulting in a deviation in the composition ofthe coating do not affect the measurement of the amount CW of thecoating according to the solution. Paper grade or grade change, inassociation with which the composition of the coating may change, doesnot either affect the measurement of the amount CW of the coating.

In some cases, the same material, e.g. calcium carbonate or kaolin, isused as filler in the paper to be coated as is used as a component inthe coating. In these cases, the same component contained by the papermay disturb the measurement of the amount of the coating. In this caseit is appropriate to measure the amount of the same component containedby the paper before the paper is coated and correct the result obtainedfrom the coated paper according to the measurement of the uncoatedpaper, i.e. the base. FIG. 1 schematically shows a third measuringdevice 43, arranged to measure the amount FC of a corresponding fillercontained by the paper before the paper is coated using for instancethrough-measurement, methods based on the absorption of radiation, knownper se to a person skilled in the art.

FIG. 2 schematically shows a first measuring device 17, which can beused to measure the amount CA of at least one component of the coatingin the paper. The operation of the measuring device 17 is based onreflection measurement at IR wavelengths, the measuring devicetransmitting infrared radiation to the object measured and measuring theradiation reflected from the object. The radiation source and thereceiver are thus arranged on the same side of the object to bemeasured. The operation of the first measuring device 17 is based on themeasurement of the ratio of the reflection intensities of the IRabsorption wavelength bands and reference wavelength bandscharacteristic of the different components of the coating and water andfibres. The ratio of the reflection intensities of the absorptionwavelength bands and reference wavelength bands corresponding to eachcomponent can be used to determine the amount of each component in thecoating remaining in the paper.

The first measuring device 17 according to FIG. 2 comprises a radiationsource 21 that generates a beam of light 22. The radiation source 21 maybe for instance a halogen lamp or another suitable radiation source forgenerating an infrared beam. The beam of light 22 is directed with afirst lens 24 to a chopper 23. Instead of the first lens 24, a lenscombination or a concave mirror can be used, which directs the beam oflight 22 transmitted by the radiation source 21 to the chopper 23. Thebeam of light 22 directed to the chopper 23 is directed further using asecond lens 25 to the object to be measured, i.e. the paper web 1 or apaperboard web and to a coating layer 10 therein. In this case, too, alens combination or a concave mirror can be used instead of the secondlens 25. The chopper 23 serves to chop the optical radiation transmittedby the radiation source 21 into light pulses so that part of the timethe chopper 23 lets through optical radiation and part of the time thechopper 23 prevents the radiation from getting through it. An essentialfeature of the operation of the chopper 23 is thus that the objectmeasured is illuminated during the illumination time using IR radiationemitted from the radiation source 21, and during the illuminationblocking time, the object measured is not illuminated with the IRradiation emitted from the radiation source. The chopper 23 ispreferably a rotating disk, rotated by an electric motor 33. Between thesecond lens 25 and the object measured, a cover 26 can be used, which isof plastic, glass, or other material that permeates the IR radiation tobe measured and which serves to protect the imaging optics particularlyunder industrial circumstances, i.e. the first lens 24, the second lens25, the chopper 23 and the radiation source 21 from being fouled.

The optical radiation emitted by reflection or scattering from theobject to be measured is collected with a third lens 27, and focused forinstance to an optical fibre or a fibre bundle 28. Furthermore, thecover 26 may also be extended between the object measured and the thirdlens to prevent the side receiving the optical radiation from beingfouled, particularly under industrial circumstances. From the fibre orthe fibre bundle 28, the optical radiation moves towards filtering anddetectors 29 a and 29 b. Since the measuring device shown by FIG. 2comprises two detectors 29 a and 29 b, the fibre bundle 28 is dividedinto two branches 28 a and 28 b. The radiation emitted from branch 28 ais filtered with a MIR filter (Middle IR) 30 a arranged in associationwith the chopper 23, and the radiation emitted from branch 28 b issimultaneously filtered with a NIR filter (Near IR) 30 b. The filtersfilter the light such that only the light essential to the measurementand in the right wavelength range reaches the detectors. The filteredMIR radiation is focused with a fourth lens 31 to the first detector 29a and the filtered NIR radiation is focused with a fifth lens 32 to thesecond detector 29 b. The chopper 23 comprises at least two MIR filters30 a, the first MIR filter letting through a wavelength range sensitiveto the coating component to be measured, the second MIR filter lettingthrough a wavelength range sensitive both to the base, i.e. the uncoatedpaper web 1, and the coating component to be measured. The choppersimilarly comprises at least two NIR filters 30 b. From the detectors 29a and 29 b, the measured signals are led either to a data processingunit inside the first measuring device 17 or to the data processingdevice 19 shown in FIG. 1 for determining the amount CA of the componentto be measured in a manner known per se to a person skilled in the art.

The measuring device shown in FIG. 2 can be used for simultaneousmeasurement of at least two different components of the coating. In theMIR range, typically referring to the electromagnetic spectral band 2500nm to 20000 nm, the amount of for instance calcium carbonate, kaolin,silicone or water can be measured. In the NIR range, typically referringto the electromagnetic spectral band 700 nm to 2500 nm, the amount offor instance kaolin, talc, gypsum, latex, starch, silicone or water canbe measured. The measurement of water can be used to determine themoisture content and further specify the measurement of the amounts ofthe other components. When several MIR and NIR filters at differentwavelength ranges are arranged in the chopper 23, the same measuringdevice can be used to substantially simultaneously measure more than twocoating components. Furthermore, the first measuring device 17 shown inFIG. 2 is only one feasible measuring device for measuring the amount CAof at least one coating component, and thus very many differentmeasuring devices can be used to measure the amount CA of at least onecomponent of the coating.

FIG. 3 schematically shows by way of example a measurement performedwith the first measuring device 17 according to FIG. 2, wherein thereflection spectrum of the stock or base paper constituting themeasuring base, i.e. the uncoated paper web 1, is shown by curve E,drawn by a continuous line, and the reflection spectrum of paper coatedwith a coating containing calcium carbonate by curve F, drawn by abroken line. The horizontal axis shows wavelength λ in micrometers (μm)and the vertical axis the absorbance of the beam of light 22. Curve Fshows the absorption peak of calcium carbonate at a wavelength ofapproximately 3.95 micrometers. Reference wavelengths suitable formeasuring calcium carbonate are 4.55 micrometers and/or 3.7 micrometers,for example. Any reference wavelengths close to the actual measuringpeak are usable as reference wavelengths. It is essential that theabsorbance of the stock and the coated paper in said referencewavelength range is the same or approximately the same. A wavelength of3.7 micrometers is particularly advantageous, since it can be used asreference also in the measurement of the amount of water. The amount ofwater is preferably measured at a wavelength of about 3.175 micrometers,for example. The measurement of the amount of water can be used tocompensate for the effect of the water in the coating and the paper inthe measurement of the amount of the coating. The amount of a pigment inthe coating is preferably used as the amount of the coating component tobe measured, since their portion in the coating is typically highest.

FIG. 4 schematically shows a second measuring device 18 usable fordetermining the composition CC of a coating to be transferred to paper.The second measuring device 18 shown in FIG. 4 is a Raman spectrometeroperating by Raman spectroscopy based on molecular vibrationspectroscopy. Raman spectroscopy is a method supporting IR spectroscopy,its advantages compared with the IR technique being less need for sampleprocessing, since Raman spectroscopy can be used for direct measurementof powdery, liquid or solid samples. A further advantage is the rapidityof the measurement and the ability to measure aqueous samples, sincewater is a weak Raman scatterer. A still further advantage of the Ramantechnique compared with the IR technique is simple calibration, whichcan be implemented as the ratio of the areas of two intensity peaks ofthe Raman spectrum.

The Raman spectrometer shown in FIG. 4 comprises a laser 34, amonochromatic illumination 35 transmitted by which is focused by imagingoptics 36 to a sample 37 to be analyzed. Lenses or fibre opticalmeasuring heads or combinations thereof, for example, are usable as theimaging optics 36. Part of the monochromatic illumination 35 transmittedby the laser 34 passes through the sample 37. Arrow 38 denotes this partof the illumination 35 transmitted by the laser 34. The rest of themonochromatic illumination 35 transmitted by the laser 34 is scatteredfrom the sample 37. Light, denoted by arrow 39 and scattered from thesample 37 and containing information about the molecular vibration inthe sample caused by the light transmitted by the laser 34, is collectedby measuring optics 40. Lenses or fibre optical measuring heads orcombinations thereof, for example, are usable as the measuring optics40. The measuring optics 40 leads the light scattered from the sample 37to a detector 41, to which the Raman spectrum descriptive of thecomposition CC of the sample 37 is imaged or recorded. A CCD camera, forexample, can be used as the detector 41. The Raman spectrometer of FIG.4 further comprises a data processing unit 42 for determining thecomposition CC of the sample 37 on the basis of the Raman spectrumimaged in the detector. Instead of the data processing unit 42, the dataprocessing unit 19 shown in FIG. 1 can also be used to determine thecomposition CC of the sample 37. FIG. 4 schematically shows only onefeasible embodiment of a Raman spectrometer. Raman spectroscopy anddifferent Raman spectrometers are known per se to a person skilled inthe art and are therefore not described in detail herein.

FIG. 5 schematically shows, on curve G, by way of example a measurementof the composition CC of the coating performed with the second measuringdevice 18 of FIG. 4, a basic level correction being performed on curveG. The horizontal axis shows the magnitude of the Raman shift in cm⁻¹and the vertical axis the Raman intensity proportioned to the largestpeak. Said peaks denote the amounts and ratios of the componentscontained by the coating to be transferred to the paper. At about 1082cm⁻¹ in the Raman shift, curve G shows a calcium carbonate peak, and atabout 1002 cm⁻¹ in the Raman shift a SB latex peak. Should the coatinginclude other components, their peaks would be shown at such points inthe Raman shift that are characteristic of them. A comparison of theheight and/or areas of the peaks with the basic level of the Ramanspectrum enables the calculation of the ratio of the differentcomponents contained in the coating and/or the amount of one or morecomponents. The determined composition CC of the coating is used in theabove-described manner for measuring the amount CW of coating appliedonto the paper.

Instead of a Raman spectrometer and Raman spectroscopy, measuringdevices based on the IR technique can also be used for determining thecomposition CC of the coating to be transferred to the paper, but thecomposition of the coating is preferably determined by Ramanspectroscopy.

Accurate measurement of the amount of the coating CW in the paper web 1under papermaking enables accurate adjustment of the amount CW of thecoating in the paper web 1. In a coating head 3 according to FIG. 1, theamount CW of the coating is adjusted by changing the position of thedoctor blade 7 relative to the paper web 1. The position of the doctorblade 7 is changed by controlling the operation of the actuatoraffecting the doctor blade 7 in accordance with a control variable COobtained from a control device 20 shown in FIG. 1. The control variableCO is determined on the basis of the difference between the amount CW ofthe coating measured in the control device 20 and the set value CW_(SP)descriptive of the target value of the amount CW of the coating.Although FIG. 1 shows the control device 20 as a device separate fromthe data processing device 19, it is naturally clear that the operationsof the control device 20 can also be implemented in the data processingdevice 19.

The drawings and the related specification are only intended toillustrate the inventive idea. The details of the invention may varywithin the scope of the claims. It is thus clear that when both sides ofthe paper web 1 are coated, the amount of the coating on both sides ofthe paper web 1 can be measured using a similar arrangement. If bothsides are coated with the same coating, the composition CC of thecoating determined by the same second measuring device 18 can be used inmeasuring the amount CW of the coating on both sides. Furthermore, thesolution presented is preferably used for continuous measurement of theamount of the coating in the paper, i.e. when the previous measuringresult is obtained, the measurement is immediately or substantiallyimmediately restarted. If the operation of the coating head is verystable, it is sufficient to measure the amount of the coating onlyrandomly or at preset intervals. The solution presented is naturallyusable in coating paper during papermaking or in coating finished paperin a coating device separate from the paper machine.

1-20. (canceled)
 21. A method of measuring the amount of a coating on apaper web, the method comprising measuring the amount of at least onecomponent of the coating on the paper web, measuring the composition ofthe coating to be transferred to the paper web, and determining theamount of the coating on the paper web on the basis of the amount of atleast one component of the coating on the paper web and the compositionof the coating to be transferred to the paper web.
 22. A method asclaimed in claim 21, comprising further adjusting the amount of thecoating on the paper web on the basis of the measurement of the amountof the coating on the paper web.
 23. A method as claimed in claim 21,wherein the amount of at least one component of the coating on the paperweb is measured by reflection measurement.
 24. A method as claimed inclaim 23, wherein the amount of at least one component of the coating onthe paper web is measured by reflection measurement based on infraredtechnique.
 25. A method as claimed in claim 21, wherein, the compositionof the coating to be transferred to the paper web is determined byreflection measurement based on infrared technique.
 26. A method asclaimed in claim 21, wherein the composition of the coating to betransferred to the paper web is determined by Raman spectroscopy basedon molecular vibration spectroscopy.
 27. A method as claimed in claim21, wherein the amount of at least one component of the coating on thepaper web is measured continuously.
 28. A method as claimed in claim 21,wherein the amount of at least one component of the coating on the paperweb is the amount of a pigment in the coating on the paper web.
 29. Amethod as claimed in claim 21, wherein the composition of the coating tobe transferred to the paper web is determined continuously.
 30. Anapparatus for measuring the amount of a coating on a paper web, theapparatus comprising a first measuring device arranged to measure theamount of at least one component in the coating on the paper web byreflection measurement, a second measuring device arranged to measurethe composition of the coating to be transferred to the paper web, and adata processing device arranged to determine the amount of the coatingon the paper web on the basis of the amount of at least one component ofthe coating on the paper web and the composition of the coating to betransferred to the paper web.
 31. An apparatus as claimed in claim 30,the apparatus further comprising a control device arranged to adjust theamount of the coating on the paper web on the basis of the measurementof the amount of the coating on the paper web.
 32. An apparatus asclaimed in claim 30, wherein the first measuring device is arranged tomeasure the amount of at least one component of the coating on the paperweb by reflection measurement.
 33. An apparatus as claimed in claim 32,wherein the first measuring device is arranged to measure the amount ofat least one component of the coating on the paper web by reflectionmeasurement based on infrared technique.
 34. An apparatus as claimed inclaim 30, wherein the second measuring device is arranged to determinethe composition of the coating to be transferred to the paper web byreflection measurement based on infrared technique.
 35. An apparatus asclaimed in claim 30, wherein the second measuring device is arranged todetermine the composition of the coating to be transferred to the paperweb by Raman spectroscopy based on molecular vibration spectroscopy. 36.An apparatus as claimed in claim 30, wherein the first measuring deviceis arranged to measure the amount of at least one component of thecoating on the paper web continuously.
 37. An apparatus as claimed inclaim 30, wherein the amount of at least one component of the coating onthe paper web is the amount of a pigment in the coating on the paperweb.
 38. An apparatus as claimed in claim 30, wherein the secondmeasuring device is arranged to determine the composition of the coatingto be transferred to the paper web continuously.
 39. An apparatus asclaimed in claim 30, wherein the second measuring device is arranged ina coating colour reservoir in a coating head, in a coating mixer, in afeed line between the mixer and the coating colour reservoir or in aseparate sample line leaving the coating colour reservoir.
 40. Anapparatus as claimed in claim 30, wherein the second measuring device isarranged in a coating colour reservoir in a coating head, in a coatingstorage or machine tank, in a transfer line between the storage andmachine tanks, in a transfer line between the machine tank and thecoating colour reservoir, in a separate sample line leaving the storageor machine tank or in a separate sample line leaving the coating colourreservoir.